In production lines of belt-like bodies or columnar bodies, a roll is sometimes used to convey products. When a certain trouble arises in a production process, defects originating from the roll are sometimes formed on a steel sheet. A method for detecting the defects will be described taking an example of a process for particularly producing a steel sheet.
In the production process of a steel sheet, periodic defects referred to as roll marks are sometimes formed. The roll marks are formed when foreign materials adhering to the roll provided in the production line or irregularities produced on the roll itself when the foreign materials bite into the roll are transferred to a steel sheet. The periodic defects are generated when the irregularities produced on the roll are transferred to a steel sheet. When generated once, the periodic defects are successively generated until the roll is exchanged or the process is improved. Therefore, it is extremely important also in terms of improvement in yield to discover the periodic defects at an early stage and to take a measure for dealing with the periodic defects.
Heretofore, many detection methods focusing on the periodicity of the periodic defects have been proposed as a method for detecting the periodic defects.
The technique disclosed in Japanese Unexamined Patent Application Publication No. 6-324005 is one of the methods utilizing the periodicity. According to that method, a target sample is measured by a defect detection sensor first, sensor output signals are subjected to synchronous addition at an expected period (length equivalent to one rotation of a final pressure roll in a steel line in JP 6-324005), and a defect signal having a period is emphasized from other noise components having no period. However, the method cannot be used for cases other than a case where the period is expected beforehand. For example, in the steel production line, the roll causing the defects is worn out, which changes the diameter of the roll in some cases. When the diameter varies, a defect development period naturally varies, and thus the above-described method is difficult to apply.
Heretofore, to deal with the problem such that the defect development period varies, some methods have been proposed.
Mentioned as a first method is a method including measuring a target sample by a defect detection sensor, subjecting sensor output signals to threshold treatment to extract a plurality of defect candidates, and comparing intervals between the plurality of defect candidates to judge that, when the intervals are in agreement with each other, the periodic defects are generated at the agreement interval as a period. However, when the method is actually applied, the following problems arise.
In an actual production line, a degree of contact between a roll and a steel sheet is not always constant. For example, the rolling reduction varies in a reduction roll. When the contact between the roll and the steel sheet is weak, a defect level is low. Therefore, a defect signal also becomes weak, and thus is not detected in some cases. Moreover, signals of slight unevenness, such as a sudden defect having no periodicity, surface roughness of a steel sheet that is originally harmless, or magnetic properties (in the case of a magnetic detection device), are detected, and they sometimes arise while being mixed with periodic defects. Therefore, the method for detecting periodicity simply by comparing the intervals between the defect candidates has problems in that the intervals between the defect candidates are not in agreement with each other due to non-defection of defect candidates or noises, such as sudden defects or over detection, and thus, periodic defects and the periodicity thereof cannot be accurately detected.
Known as a second method for dealing with the problem such that the defect development period varies is a detection method utilizing autocorrelation (e.g., Japanese Unexamined Patent Application Publication No. 58-156842).
The distance between peaks obtained based on computation results of autocorrelation represents a period of periodic signal components. Therefore, even when the period of the periodic signal components contained in a signal series to be processed is unknown, only the periodic signal components can be selectively extracted from the signal series buried in noise. However, the detection method utilizing autocorrelation also has a problem in that, when many noise components are contained in sensor output signals from a target sample, a periodicity judgment accuracy decreases. The detection method also has a problem in that, when a detection sensitivity is lowered so as to suppress the over detection, minor signals from minor defects cannot be detected.
Mentioned as a third method for dealing with the problem such that the defect development period varies is a detection method disclosed in Japanese Unexamined Patent Application Publication No. 2006-105791, for example.
The detection method includes successively picking up images of the surface of a moving belt-like body or the like, cutting out a template image T from the picked-up images, and comparing image similarity between the template image T and a target image G whose length in the longitudinal direction is longer than a target roll circumferential length to detect periodicity. According to the method, to accurately determine a defect period, correlations of not only defect parts but base-design parts of parts free from defects are calculated. Therefore, the image similarity is compared in the range equal to or more than one roll rotation. According to that method, in a stage of evaluating the image similarity between the template image and the target image, the similarity is evaluated by gradually shifting a relative position of the mutual images. Therefore, the method can be applied to the case where the diameter of the roll gradually varies due to abrasion. However, that method has problems in that the method cannot be applied to rolls other than a roll on which a base pattern is formed, i.e., a final reduction roll before inspection, because the base pattern of parts free from defects is utilized for obtaining period information.
In an actual production line of a steel sheet, it is necessary to detect not only defects originating from a roll generated in the final reduction roll before inspection but such defects that have been generated before. Specifically, in the case of cold rolling, it is necessary to detect defects in a reduction roll in one stage or two stages prior to the final reduction roll, defects in a roll in an annealing furnace prior to a final temper rolling roll in a CAL line, defects originating from the roll generated during cold rolling, etc. Therefore, it is necessary to detect defects generated in a plurality of rolls having different diameters. However, the detection method of JP 2006-105791 cannot be applied to the case.
It could therefore be helpful to provide a device for detecting periodic defects of a belt-like body or the like which can be used even when the defect development period varies and which can judge, with a high degree of accuracy, periodic defects generated not only in a reduction roll but in a plurality of rolls having different diameters, particularly even when the defects are minor, and a method therefor.